[FIX] #802: Viewport is not correctly updated in Follow Mode

Changed the viewport alteration on the follower's side from the
(inaccurate) ITextViewer.revealRange method to
ITextViewer.setTopIndex. In the old version the viewport was altered in
two different methods, I'm combining them into one method.

Change-Id: Ic82ce9d7fe4f0f8e5a70415a963eeaabeced8de6
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1220
Tested-by: Jenkins CI
Reviewed-by: Andrej Szaffranietz <a.szaffranietz@gmail.com>
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Nils Bussas <nils.bussas@gmail.com>